This invention relates to friction welding and in particular concerns friction welding thin-walled structures.
The invention finds particular application in the manufacture of gas turbine aero-engine casings which hitherto have been manufactured from titanium, nickel or steel forgings. The manufacture of thin walled components such as aero-engine casings from metal forgings is particularly expensive in terms of material wastage and machining time. Typically ninety five percent of forging material is removed during the machining of an engine casing forging. This low material utilisation is a consequence of the forging process since the size of load bearing reinforced features such as bosses and the like on the casing surface determine the wall thickness of the forging that is necessary for correct material flow during the forging process. For example, a forging for an engine casing having a 25 mm (1 inch) diameter boss on its surface requires a minimum wall thickness of at least 25 mm to ensure correct material flow in the region of the boss during forging.
Bosses are a common feature on gas turbine aero-engine casings since they are used extensively for mounting pipes and vane spigots, for example which pass through apertures in the casing. For instance, the intermediate compressor casing for the V2500 engine manufactured by International Aero Engines comprises over two hundred bosses for locating and supporting the radially outer spigots of the variable geometry compressor vanes.
Boss diameters of 25 mm or more are common on casings having a wall thickness in the region of 2-5 mm and the resultant material wastage and machining time adds considerably to the manufacturing cost of thin-walled engine casing structures and adds significantly to the lead time of the machined component.
One proposal to address this problem has been to manufacture gas turbine aero-engine casings from sheet material using highly accurate fusion welding techniques such as electron beam welding. However, fusion welded bosses have a number of drawbacks particularly in terms of joint strength, mechanical integrity and the cost associated with non-destructive (NDI) weld inspection In this respect fusion welded bosses are usually unsuitable for gas turbine aero-engine casing applications and the manufacture of casings from forgings has hitherto been preferred.
Friction welding has also been proposed for joining bosses to engine casings manufactured from thin sheet material. Friction welding is the welding method of choice in many welding applications since parent material strength can be achieved at the weld joint with little or no heat affected zone. Attempts at friction welding bosses to thin walled structures such as engine casings have not been wholly successful however, since the high forging loads generated cannot always be supported by the thin walled casing when the casing material becomes plastic during the welding process. This results in the boss element punching through the thin walled casing, in a process known as xe2x80x9cburn throughxe2x80x9d, before a satisfactory weld is achieved.
These constraints limit the size of boss that can be welded to a casing of a specified wall thickness. This is a significant obstacle where large diameter bosses are required, for instance on gas turbine areo-engine compressor casings where large diameter vane spigot bosses are used for supporting the aerodynamic loads that act on variable geometry compressor vanes.
According to an aspect of the invention a friction welded component comprises at least two structural support members spaced apart and connected by a plurality of first and second studs, each of the first studs being friction welded at one end to a first of the support members and at the opposite end thereof to one end of a respective second stud, whereby each of the second studs are friction welded to a respective first stud and a second of the support members along a continuous annular friction welded joint.
It is to be understood that the terms xe2x80x9cstudxe2x80x9d and xe2x80x9cbossxe2x80x9d are used interchangably throughout this description, with the term xe2x80x9cstudxe2x80x9d being used to refer to any stud like component suitable for friction welding to form a xe2x80x9cbossxe2x80x9d on the surface of a structural member.
The above aspect of the invention readily enables bosses in the form of studs to be friction welded to thin walled structures such as engine casings since bosses having cross-sections smaller than would otherwise be necessary may be friction welded to thin walled engine casing structures to support relatively high loads. The secondary support member increases the bending stiffness of the welded structure in the region of the bosses which enables smaller diameter bosses to be used. This readily permits load supporting bosses to be friction welded to thin walled structures in applications where hitherto it has not been possible because of constraints such as xe2x80x9cburn throughxe2x80x9d due to the higher forging loads associated with larger diameter studs or bosses. The continuous annular friction welded joint readily enables the weld zone between the welded components to be minimised since only limited local frictional heating is required to form the welded joint.
In this aspect of the invention the first and second studs may be considered to constitute the web and the first and second members the respective flanges of an I-section beam.
Preferably, the said continuous annular weld has a divergent cross-section. This readily enables the first and second studs to be joined along the single continuous surface and thereby minimise the cross-section of the welded joint between the studs and second support member.
In preferred embodiments, the continuous annular weld diverges in the direction from the first support member towards the second support member.
Preferably, the continuous annular weld has a frusto-conical configuration. This readily enables rotary friction welding methods such as inertia welding to be used to join the first and second studs to each other and the second member.
In preferred embodiments, the second stud protrudes from the second support member in the direction away from the first support member.
Preferably, each pair of welded first and second studs include a throughbore extending through the first and second support members and the said studs. In this respect the welded bosses so formed may be used to support load bearing members passing through the first and second support members.
In preferred embodiments, the said first support member comprises a primary structural support member and the said second support member comprises a secondary reinforcement member.
Preferably, the second support member comprises at least one link element extending between adjacent first and second welded stud pairs.
In preferred embodiments, the first support member comprises a primary casing and the second support member comprises a containment casing for containing projectiles piercing the said primary casing. Thus the support members may comprises a layered structure for containment purposes, for example for containing high energy projectiles such as fragments of rotating components or the like piercing the first support member.
The friction welded component may comprise a plurality of containment casings spaced apart and substantially parallel to the primary casing. This can further improve the above mentioned advantages.
In preferred embodiments, the first structural member comprises a generally cylindrical gas turbine engine casing and the said second support member is disposed on the radially outer side of the said engine casing. In gas turbine areo-engine applications containment of fractured engine components is a significant design consideration.
According to another aspect of the invention there is provided a method of friction welding comprising the steps of;
friction welding a plurality of first studs at one end thereof to a first structural support member;
positioning a second structural member having a plurality of apertures adjacent the opposite ends of the first studs, the apertures being positioned in register with the said first studs;
friction welding a plurality of second studs to the said second support member, each of the second studs extending through a respective aperture and being friction welded along a continuous friction weld surface to the second support member and a respective first stud.
The continuous weld surface proposed by this aspect of the invention readily enables a localised joint having a relatively small cross-section to be formed between the components since frictional heating, forging and welding occurs in a uniform manner over the whole of the weld contact surfaces.
An advantage of using friction welding to attach the bosses in this embodiment is the relatively low amount of heat input into the fabricated structure which can minimise distortion, buckling etc of sheet metal casings, for example.
Preferably, the step of friction welding the second studs comprises the step of simultaneously welding the second stud to the respective first stud and aperture along the continuous friction weld surface of the second stud.
In preferred embodiments, the end of the first studs to be welded to the second studs are provided with a recess for receiving the end of the second stud to be welded.
Preferably, each recess and respective adjacent aperture define a continuous surface which contacts the continuous weld surface of the second stud.
In preferred embodiments, the said continuous surface of recess and aperture and the continuous surface of the second stud have corresponding frusto-conical configurations.
Preferably, the step of friction welding the said second studs to the said first studs comprises rotary friction welding.